The present invention relates to a semiconductor memory, and more particularly, to a technology for sensing data having a multi-level in a non-volatile memory such as a phase change memory.
Recently, a phase change random access memory (PRAM) is drawing attention as a next generation semiconductor memory and is implemented using a phase change material.
The PRAM includes a cell storing data using a germanium antimony telluride (Ge2Sb2Te5), referred to as GST hereafter, which is one of phase change materials. The GST as a phase change material changes to crystalline or amorphous state according to the change of temperature and the amount of current, and the PRAM stores data “0” or “1” according to the phase change.
Upon a write operation, when a current flows on the GST, the GST changes to the crystalline or amorphous state. The phase change of the GST occurs due to the Joule heating generated by a current applied to a cell.
Upon the write operation, when the GST is heated to above its melting temperature by a write current IWRITE and then is rapidly cooled, the phase of the GST changes to the amorphous state, and stores data “H” corresponding to the phase change. The amorphous state is referred to as a reset state.
Upon the write operation, when the GST is heated to above the crystallization temperature by the write current IWRITE and then is cooled after being maintained for a certain period, the phase of the GST changes to the crystalline state and stores data “L” corresponding to the phase change. The crystalline state is referred to as a set state.
As described above, although the PRAM stores a single data having one of two states, that is, high resistance and low resistance, the PRAM can also have four-level resistance distribution by precisely controlling the resistance distribution in the write operation.
In the non-volatile memory such as the phase change memory described above, technologies for reading or writing multi-level data have been developed in order to store much more data using restricted resources.
In order to read or write multi-level data, a method for sensing the multi-level of data is required. In most cases, different levels of reference voltages and an apparatus including sense amplifiers with respect to each reference voltage are used to sense the multi-level data.
Specifically, the sensing of four-level data requires three reference voltages and three sense amplifiers. That is, the multi-level sensing apparatus for sensing the four-level data determines which one of the four levels data read through outputs of the three sense amplifiers should be read.
However, the semiconductor chip has been developed to have higher integration density in a restricted area. To meet such a trend, the multi-level sensing apparatus is required to have higher integration density.